MRI systems are now in common use for a variety of medical and scientific imaging applications. The following generally relevant commonly-assigned U.S. Patents (incorporated by reference herein) describe some exemplary conventional MRI systems:
U.S. Pat. No. 4,297,637 to Crooks et al; PA0 U.S. Pat. No. 4,318,043 to Crooks et al; PA0 U.S. Pat. No. 4,471,305 to Crooks et al; PA0 U.S. Pat. No. 4,599,565 to Hoenninger et al; and PA0 U.S. Pat. No. 4,684,891 to Feinberg.
The following commonly assigned copending patent application generally relates to calibration of MRI systems:
Serial No. 181,440 of Ching filed Apr. 14, 1988 entitled "MRI Compensated For Spurious NMR Frequency/Phase".
The following principles of NMR are well known. All nuclei with an odd number of protons or neutrons behave, in effect, like small magnets. When placed in a steady external magnetic field, the magnetic axes of such nuclei precess at an angle about the imposed field axis at the so-called Larmor frequency. The Larmor frequency is related to the magnetic field of the nucleus by the so-called gyromagnetic ratio characteristic of the particular type of nuclei. As is well known, the direction of the net angular momentum or "spin" of a group of nuclei (and thus their net magnetic axis) can be reoriented with respect to the external magnetic field by electromagnetic signals having a frequency equal to the Larmor frequency. The electromagnetic signal produces a stationary magnetic field in the rotating frame of reference to nutate (reorient) the net spin resonance (Larmor frequency) nuclei by an amount determined by the amplitude and duration of the electromagnetic signal.
Over a period of time, after removal of the electromagnetic signal, many magnetic moments will realign parallel to the external magnetic field. As nuclear realignment occurs, the relative phases of the individual spins begin to diverge as some nuclei precess faster and some slower then the central Larmor frequency. Thus, there is a gradual "dephasing" of the individual nuclear spins and a consequential loss of phase coherence. In a perfectly uniform magnetic field, such dephasing results from natural processes which cause nuclei to exchange energy with each other. The length of time that such dephasing takes to occur is related to the "spin-spin", or transverse, relaxation time constant T.sub.2. During realignment, the nuclear moments also lose energy to their surroundings and thus relax, orienting parallel to the external magnetic field. The "spin-lattice", or longitudinal, relaxation time T.sub.1, is related to this time of relaxation.
As is also well known, nuclear spins initially aligned with the external magnetic field and then reoriented transverse to the initial direction induce a characteristic RF signal in an appropriately oriented coil connected to an RF signal receiver. Initially upon reorientation, a relatively strong voltage is induced in the receiver coil which gradually decreases in amplitude due to field inhomogeneity and to energy exchange between spins. This signal is called the "free induction decay" (FID). As is also well known, a "spin echo" or subsequent representation of the FID can be generated by bringing the respective spins back into phase coherence through use of a so-called "pulse sequence." For example, if at a time .tau. after the nuclear spins are "flipped" or reoriented (for example, 90.degree. with respect to initial direction) by a first electromagnetic pulse of appropriate frequency magnitude and duration, and then another electromagnetic signal of appropriate frequency, magnitude and duration is applied to effect 180.degree. nutation of the nuclear spins (hereinafter referred to as "180.degree. pulse"), the accumulation of further phase deviation for individual nuclear spins cause all of the individual spins to, at time 2.tau., again come into phase coherence to produce a so-called "spin echo" of the FID.
Because the RF pulses "flip" the nuclei rotation axis, the reorientations (nutations) in the nuclei precession angle they generate are commonly referred to as "flip angles."
As is also well understood by those skilled in this art, calibration of the RF transmitter and associated coils and components is critical to providing desired results based upon the phenomenon discussed above and/or based on other MRI phenomenon. The amplitude and duration of an applied RF pulse determines the nutation angle imparted to the nuclei precession. Thus, to obtain desired nutation angles it is necessary to generate RF pulses having corresponding desired durations and amplitudes. However, such RF pulse excitation of an object (e.g., a human patient) to be imaged is typically provided by applying the RF pulse to an RF coil closely coupled to the object--so that RF coil loading (and thus resulting radiated RF amplitude) depends on the position, size and other parameters of the object. Consequently, it is typically necessary to recalibrate the RF transmitter for each image acquisition (i.e., "study" or set of scans). Moreover, if the patient is moved, the Quality Factor (Q) and loading of the coil changes and the RF amplitude within the particular area of interest within the patient's body thus also changes. This typically requires the RF transmitter output level to be recalibrated for each new patient and also each time the patient is moved with respect to the RF coil in order to ensure desired nutation angles are being obtained for given RF transmitter output levels.
It is possible to reduce or eliminate the necessity for repeated RF transmitter output recalibration by "de-Qing" the RF coil (so that coil loading is less affected by the positioning and other parameters associated with the patient). Unfortunately, a low Q RF coil uses RF power less efficiently, and the high magnetic field intensities provided by most superconducting magnet type MRI systems therefore require high Q RF coils.
In order to increase the patient throughput of an MRI system and for other reasons (e.g., to minimize the apprehension and anxiety some patients suffer because of long scanning times), it is important to perform the requisite RF tuning and calibration as rapidly as possible. The following is a somewhat representative listing of documents relating to decreasing the time required for RF calibration:
van der Muelen, P. and van Yperen, G. H., Proceedings, Society of Magnetic Resonance in Medicine Fourth Annual Meeting 1129 (1985);
Sattin W., "A Rapid, High Signal-to-Noise RF Calibration System", Proceedings, Society of Magnetic Resonance in Medicine Seventh Annual Meeting 1016 (1988);
Perman, W. H., Bernstein, M. A. and Sandstrom, J. C. "A Method For Correctly Setting the Flip Angle", Magn. Reson. Med. 9 16 (1989);
Woessner, D. E. "Effects of Diffusion in Nuclear Magnetic Resonance Spin-Echo Experiments", J. Chem. Phys. 34 2057 (1961);
U.S. Pat. No. 4,788,501, Leroux et al (1988);
U.S. Pat. No. 4,739,267, Leroux et al (1988).
Generally, known techniques for setting RF transmitter level include:
seeking a maximum spin echo signal in a sequence;
evaluating a ratio of signals in a three or more pulse sequence as a measure of flip angle; and
cancelling a signal from a sequence of one or more pulses.
The first method is the simplest and generally takes the longest time to perform. Not only is a search for a maximum a time-consuming endeavor, but accuracy demands a delay of several T.sub.1 relaxation times between repetitions. Commonly, the search for a maximum is automatically performed by obtaining NMR responses (e.g., four "spin echo" responses) from several different RF excitation levels (waiting at least a T.sub.1 relaxation time between different excitation cycles) and then applying conventional maximum-determining algorithms (e.g., curve fitting) to the resulting data. The excitation/acquisition process must typically be repeated five or six times before the algorithm converges accurately, and thus typically requires at least 45 seconds to perform. The method must be repeated each time the patient is moved significantly with respect to the RF coil, and thus may introduce significant delay into complex studies requiring different patient orientations.
The Muelen and Sattin documents cited above teach a calibration method employing the first Hahn echo and stimulated echo from a three RF pulse sequence to measure the flip angle. Based on the estimate of the flip angle, a new level is chosen and the search continues. This is generally a more rapid procedure due to predicted nature of the duration. Generally once such methods are within range, the results they provide converge in two or three iterations. Muelen teaches generating an intensity ratio from a combination of the stimulated echo and first spin echo of a three pulse sequence of identical pulses, this ratio exhibiting no T.sub.2 dependence and a weak T.sub.1 dependence. Muelen teaches calculating the nutation angle from this intensity ratio. However, the T.sub.1 dependence introduces error into the calculation.
Techniques which adjust the flip angle by seeking a cancellation in signal include an older method of setting a 180.degree. pulse by minimizing the FID as well as the more recent Woessner publication cited above which sets a 90.degree. pulse by minimizing an echo of the three pulse sequence.
In contrast to the methods described above, the present invention actually provides an expression by which flip angle can be calculated substantially independently of T.sub.1 and T.sub.2. The present invention provides a robust technique for calibrating RF transmitter parameters in an MRI system. This technique can discriminate flip angles over a wide range, and is more rapid than prior art calibration techniques while avoiding systematic errors due to relaxation during the pulse sequence.
Briefly, the present invention provides a technique for adjusting RF transmitter levels based on flip angles actually calculated from received responses. In the preferred embodiment, a three pulse sequence (e.g., .theta.-.tau.-.theta.-3.tau.-.theta.) is transmitted to generate plural NMR responses (where each of the three pulses has the same amplitude and duration and thus provides the same nutation angle .theta.. For example, the plural responses can include a "stimulated echo" response S.sub.1 and "spin echo" responses E.sub.2, E.sub.13 and E.sub.23.
It is possible to use various combinations of these echo responses to provide simplified expressions for the flip angle. In particular, we have discovered that certain ratios of the echoes are independent of both relaxation times T.sub.1 and T.sub.2, are not restricted with respect to the range of flip angles, and provide accurate results over the 0.degree. to 180.degree. flip angle range generally of interest. These ratios can be used to calculate (estimate) flip angle resulting from a particular RF transmitter output level and may thus be used to automatically adjust (or to assist an operator in manually adjusting) the transmitter level for particular desired flip angles. Iterations of the excitation/acquisition sequence can be performed without waiting for relaxation because the calculations are substantially independent of relaxation times--and may therefore, for example, provide accurate automatic RF level calibration within on the order of three to five seconds (as compared to forty-five seconds required by the typical prior art maximum seeking algorithm).